What is von Willebrand's disease?
Von Willebrand's disease is one of the most common hereditary bleeding disorders in dogs and has been identified in more than 50 different breeds. Breeds with an increased incidence of von Willebrand's disease include the Doberman Pinscher, Manchester Terrier, Pembroke Welsh Corgi, French Poodle, Shetland Sheepdog, and Scottish Terrier. Cats are rarely affected.
Clinical signs that may be noted with von Willebrand's disease include bleeding from the gums, recurrent nosebleeds, and bloody urine or feces. Excessive bleeding following a simple procedure like clipping toenails may be noted. Surgery and minor physical trauma may also result in excessive blood loss.
See the handouts “Von Willebrand’s Disease in Dogs” and “Von Willebrand’s Disease in Cats” for more information about this disease.
What causes von Willebrand's disease?
Von Willebrand's disease is caused by a deficiency of von Willebrand's factor, a blood protein that is required for optimal clotting of blood. Von Willebrand's factor enhances the adherence or 'stickiness' of platelets to sites of blood vessel injury, as well as stabilizing one of the other blood clotting factors (factor VIII) in circulation so that this factor is not lost prematurely. When the amount of von Willebrand's factor is reduced, the dog is unable to deal effectively with blood vessel injuries because an effective platelet 'plug' cannot be maintained at the site of the injury.
How can von Willebrand's disease be diagnosed?
Initial tests include an evaluation of a complete blood count (CBC) to ensure that other causes of excessive bleeding, such as reduced platelet numbers, are not contributing to the clinical signs. In most cases, platelet numbers will be within the normal reference range in dogs with von Willebrand's disease.
"Initial tests include an evaluation of a complete blood count (CBC) to ensure that other causes of excessive bleeding are not contributing to the clinical signs."
A buccal mucosal bleeding time (BMBT) may be performed in the clinic. This test evaluates the ability of platelets to form a platelet plug at the site of a small cut in the upper lip. A spring-loaded cassette is used to produce a small, precise cut and the length of time required for bleeding to stop is evaluated. This time may be lengthened by both decreased platelet numbers and by decreased platelet function. Therefore, the BMBT is usually reserved for dogs that have normal platelet numbers but questionable platelet function. The value of the BMBT for dogs suspected of having von Willebrand's disease is unknown because relatively large decreases in von Willebrand's factor are needed before the BMBT is significantly increased.
Next, a coagulation panel (including the prothrombin time and activated partial thromboplastin time) may be evaluated to rule out reductions in other clotting factors as a cause of excessive hemorrhaging. Such coagulation panel testing requires a single blood sample that is sent to a veterinary referral laboratory. The results of coagulation panel testing will be normal in dogs with von Willebrand's disease.
Dogs with a history of unexplained episodes of bleeding that have normal platelet numbers and a normal coagulation (clotting) profile are candidates for specific von Willebrand's factor testing.
Immunoassays (enzyme-linked immunosorbent assay/ELISA) can be used to determine the concentration of von Willebrand's factor in a blood sample. Immunoassays are performed at a veterinary referral laboratory using specific proteins, called antibodies, to capture or bind von Willebrand's factor for measurement. Von Willebrand's disease is most commonly diagnosed by immunoassay.
"Von Willebrand's disease is most commonly diagnosed by immunoassay."
vWF collagen binding assay assesses the ability of vWF to bind to collagen fibers. It determines the ability of the vWF in a dog’s blood to bind to collagen similarly to how it would bind to blood vessels.
Genetic testing for the von Willebrand's gene is available at highly specialized referral laboratories and is typically reserved to screen dogs used for breeding programs.
